1. Field of the Invention
The present invention relates to a misfire detection apparatus for an internal combustion engine capable of detecting combustion or misfiring by detecting a change in the amount of ions generated by combustion of an air fuel mixture in the internal combustion engine.
2. Description of the Related Art
It is generally known that ions are generated when an air fuel mixture is burnt or combusted in cylinders of an internal combustion engine. Thus, the ions thus generated can be detected and observed as an ionic current by means of a probe which is installed in each of the cylinders and on which a high voltage is impressed as a bias voltage. That is, the occurrence of combustion or misfiring in all the cylinders can be individually detected or determined by detecting the presence or absence of such an ionic current. However, soot might sometimes adhere to the electrodes of a spark plug installed in each cylinder as a result of combustion of the air fuel mixture depending upon the operating condition of the internal combustion engine.
For instance, assuming that the bias voltage is 100 V and the insulation resistance of the spark plug adhered by soot is 5 MΩ, a leakage current of 20 μA flows. As a result, a leakage current flows into an ionic current detection part while monotonously attenuating at a prescribed time constant in accordance with the impression of an ignition pulse, as shown in the FIG. 8. In addition, after the start of discharging of an spark plug, an ionic current generated by combustion of an air fuel mixture flows while being superimposed on the leakage current that decreases monotonously and gradually according to a time constant CR which is determined by a high resistance of the sooted ignition plug and a capacitor component C of a biasing circuit.
FIG. 9 is a constructional view of a known misfire detection apparatus for an internal combustion engine disclosed in Japanese Patent Laid-Open No. 2001-90647. In FIG. 9, an ionic current detection part in the form of a biasing circuit 1A outputs an ionic current which is superimposed by a monotonously decreasing leakage current upon occurrence of a leakage current. A bottom level of the output from the biasing circuit 1A after being masked by a mask circuit 4A is maintained as a bottom hold value, based on which a threshold for ionic current determination is set. A waveform shaping circuit (hereinafter referred to as a bottom hold waveform shaping circuit) 6A performs waveform shaping in accordance with the bottom hold value, and makes a comparison between the level of the ionic current input from the biasing circuit 1A after the lapse of a predetermined mask time and the threshold so as output a combustion pulse.
Next, the operation of the above-mentioned known misfire detection apparatus will be described while referring to FIG. 10(A) through FIG. 10(D). When an ignition pulse IB (see FIG. 10(A)) is impressed, a leakage current flows into the ionic current detection part in the form of the biasing circuit 1A while monotonously attenuating at a prescribed time constant. The ionic current (see FIG. 10(B)) detected by the biasing circuit 1A is input to the mask circuit 4A where it is waveform shaped at a fixed threshold to form a pulse which is then cut for a predetermined period of time to provide a mask signal. A bottom value after removal of the masking is maintained as a bottom hold value, based on which a threshold for ionic current determination is set (see FIG. 10(C)).
The ionic current output from the biasing circuit 1A is input to the bottom hold value waveform shaping circuit 6A, where it is compared with the bottom level of the ionic current after removal of the masking. When the level of the ionic current exceeds the threshold, the bottom hold value waveform shaping circuit 6A outputs a combustion pulse. As a result, combustion in an explosion cylinder can be detected irrespective of the magnitude of the leakage current by waveform shaping the ionic current superimposed on the leakage current into the combustion pulse to be output (see FIG. 10(D)).
However, in the known misfire detection apparatus as described above, there might be sometimes generated an ionic current due to combustion which is superimposed on a leakage current and which does not exceed the bottom hold value, depending upon a certain operating condition of the internal combustion engine, as shown in FIG. 8.
It is impossible to determine the occurrence of combustion based on the bottom hold threshold if there is no range in which an ionic current increases, as indicated by t11 in FIG. 10(D). In this case, there arises a problem that the occurrence of combustion is unable to be detected and hence is mis-detected as a misfire.